Filter Ghostel copy text through buffer substring

[emil-e]

Summary

• install a Ghostel buffer-local filter-buffer-substring-function so generic copy paths remove renderer soft-wrap newlines
• make ghostel-readonly-copy delegate to kill-ring-save
• add a regression test for kill-ring-save copying soft-wrapped text

Verification

• emacs --batch -Q -L lisp -L test -l test/ghostel-test-helpers.el --eval "(byte-compile-file \"lisp/ghostel.el\")"
• emacs --batch -Q -L lisp -L test -l ert -l test/ghostel-test-helpers.el -l test/ghostel-render-test.el --eval "(ert-run-tests-batch-and-exit '(or ghostel-test-filter-soft-wraps ghostel-test-kill-ring-save-filters-soft-wraps))"
• emacs --batch -Q -L lisp -L test -l ert -l test/ghostel-test-helpers.el -l test/ghostel-mouse-paste-test.el --eval "(ert-run-tests-batch-and-exit '(or ghostel-test-readonly-copy-exits-when-fast-exit-enabled ghostel-test-readonly-copy-no-exit-when-fast-exit-disabled ghostel-test-readonly-copy-deactivates-mark))"